De novo synthetic models or ``maquettes'' of prototypical electron transport
membrane proteins based on a four-helix bundle motif with selected
positions for liganding one or more prosthetic groups (e.g., heme groups)
have been designed and synthesized by Prof. P. L. Dutton's research group
at Penn. Correlated structural and electrochemical studies of such
maquettes depend on our ability to orient them vectorially at an interface, preferably
with the axis of the bundle more perpendicular than parallel to the interface. While
sedimentation and NMR methods suggest that the dihelices spontaneously
assemble to form a native four-helix bundle in bulk aqueous solution, X-ray
reflectivity studies have shown that the bundle is unstable to an air-water
interface with both alpha-helices of the dihelix lying in the plane of the
interface irrespective of the surface pressure in a Langmuir monolayer.
Subsequently, a palmitic acid hydrocarbon chain (C16) was covalently linked
to the amino-terminus of each helix of the dihelix to make the maquette a
better amphiphile. X-ray reflectivity studies of this palmitoyl-derivative
of the maquette itself and of its binary mixtures with palmitic acid have
shown that the alpha-helices of the dihelix are oriented approximately perpendicular to the
air-water interface at higher surface pressures within the Langmuir
monolayer and that these monolayers can be transferred to an alkylated
inorganic substrate retaining their orientation relative to the
interface plane.